1. Field of the Invention
This invention relates to multiplex communication systems wherein a plurality of units are connected in series by means of a closed-loop link operating in the time-division multiplex mode with recurrent frames of equal duration. The invention relates more particularly to a synchronization system for causing the time taken by the time-division channels to circulate around the closed-loop link to remain constant and for supplying timing signals to the units interconnected by said link. The invention is particularly suitable for synchronizing closed-loop switching networks capable of switching both voice signals and data signals, such as the network described in co-pending European patent application No. 82-4300420 and assigned to the assignee of this application.
2. Prior Art
In switching networks wherein a plurality of switching modules are connected in series by means of a closed-loop link operating in the time-division multiplex mode and capable of switching voice signals, the regulation of the propagation times of the signals traveling on said link is of great importance, as the following will show. In such networks, the time available is divided into recurrent frames of equal duration and a multiplex message carrying a number of time-division channels and having a duration equal to that of one of said frames circulates continuously on the closed-loop link. To establish a connection between a transmitting station and a receiving station, a time-division channel is allocated to the receiving station and the transmitting station transfers thereto via said channel a sample of the voice signal at the occurrence of each frame. Since the voice signal samples are produced by the transmitting station at a regular rate, usually 8 KHz, it is essential that the time-division channel allocated to the receiving station be available to the transmitting station at the same rate. Accordingly, it is necessary to provide a synchronization system to cause the propagation time of the multiplex message around the closed-loop link to remain constant and equal to the duration of a frame. Obviously, it is also necessary to ensure that the various modules of the switching network are provided with suitable timing signals to enable them to intercept the incoming time-division channels.
U.S. Pat. No. 3,681,759 describes a loop synchronizing device for a multiplex communication system in which the time-division channels carry data at different speeds. The device is mainly comprised of a high-speed buffer, a medium-speed buffer and a low-speed buffer, into which the contents of the input time-division channels are selectively written under the control of an input timing signal and from which they are read under the control of an output timing signal. Such a device is extremely complex and unsuitable for synchronizing voice signals on a loop.
U.S. Pat. No. 4,071,706 describes a loop synchronizing device that is much simpler than the preceding one but is designed to be used with a loop on which data bursts are traveling, rather than with a loop link operating in the time-division multiplex mode. This device is mainly comprised of a buffer into which the incoming data bursts are written and out of which they are read under the control of the same timing signal. Synchronization is achieved by adjusting the length of the data bursts retransmitted by the device. Such a device is not usable in time-division switching networks capable of switching voice signals.